How would you design automated credential rotation to handle the overlap window safely, so the application never experiences an auth failure?

Never deactivate the old key at the same moment the new key is created — generate the new key first, update the application's configuration to use it, confirm the application is successfully authenticating with the new key, and only then deactivate (not immediately delete) the old key, keeping it valid for a defined grace period in case any in-flight process still references the old value, before finally deleting it.

AWS allows two active access keys per IAM user simultaneously: this is the mechanism that makes safe rotation possible at all — since both old and new keys can be valid at the same time, there's no inherent moment where the application must be without a working credential, as long as the rotation process is sequenced to take advantage of this rather than doing an instant swap.

Sequence: create new, verify, then retire old — never the reverse: generate the new access key while the old one remains fully active; update the application's configuration (environment variable, secrets manager entry) to the new key; actively verify the application is successfully authenticating with the new key (not just "the config was updated" but confirming actual successful API calls using it); only after that verification succeeds, deactivate the old key.

Deactivate before deleting, with a grace period: AWS lets you deactivate a key (making it unusable but not permanently destroyed) before deleting it outright — keeping a deactivated-but-not-deleted old key for a defined grace period (a day, a week, depending on how confident you are nothing still references it) provides a fast rollback path if something unexpected surfaces still depending on the old key, without needing to generate an entirely new key again from scratch.

Handle configuration propagation delay explicitly: if the application reads credentials from a secrets manager or config store rather than being restarted with new environment variables directly, there can be a delay between "the new key exists" and "every running instance of the application has actually picked it up" (cache TTLs, rolling restarts) — the verification step needs to confirm the new key is actually in active use across all running instances, not just that the secrets store was updated, before proceeding to deactivate the old key.

Automate the whole sequence, with explicit health checks between steps: a fully automated rotation (a scheduled Lambda function, for instance) should include an explicit health-check step between "update to new key" and "deactivate old key" — checking real application health/error-rate metrics, not just assuming success — with the automation halting and alerting (rather than proceeding to deactivate) if that health check doesn't pass, since deactivating the old key before confirming the new one truly works would defeat the whole purpose of the overlap window.

• AWS allows two simultaneously active access keys per IAM user — the mechanism that makes safe, zero-downtime rotation possible.
• Sequence strictly: create new, verify it's actually working, only then deactivate the old — never deactivate before verification.
• Deactivate (don't immediately delete) the old key, keeping a grace period as a fast rollback path if something unexpected still depends on it.
• Explicitly verify propagation across all running application instances, not just that a secrets store was updated, before proceeding to deactivate.

• AWS: Rotating access keys
• AWS: IAM access keys best practices